This invention relates generally to an automatic threshold circuit for determining the optimal threshold level set-point for a signal processing device and can be applied specifically to applications for dynamic noise reduction, where the circuit will automatically determine the threshold set-point based on the relative level of background noise. The intelligent automatic threshold circuit can be applied to many types of signal processors and its advantages will specifically be demonstrated as used with non-complimentary noise reduction systems incorporating both dynamically-controlled filtering and downward expansion.
The setting of the threshold is one of the most crucial aspects of a properly functioning non-complimentary or single-ended noise reduction system, as an incorrectly set threshold produces very undesirable results. For example, if the threshold is set too high, the dynamic filter will not operate properly and an audible loss of high frequency content will result. Low level high frequency information will not cause the dynamic filter to open up at all in this situation. If the threshold is set too low, the noise floor of the audio source itself will tend to hold the dynamic filter open and a perceived "breathing" effect will occur with any subtle change in signal level, even in the mid-band information. Therefore, it is important for proper operation of a single-ended noise reduction system that the threshold be set properly for the level of the background noise relative to the signal level.
Simplistic attempts to provide a threshold control circuit that has been de-sensitized to the changes in background noise, which would reduce the requirement for user adjustments of the threshold with varying audio sources, have been used in some signal processors. One such circuit is disclosed in my U.S. Pat. No. 4,893,099 in which the filter control for dynamic filtering is derived after the output of a compressor. By having a compression circuit in front of the filter control block, the filter control can be de-sensitized to changes in the input signal. This will increase the range in which the dynamic filter will operate with changes of the audio source without requiring an adjustment of the threshold. However, when utilizing this method, the filter responds to the envelope of the compressor output signal, and with a sudden transient the filter would increase it's sensitivity dramatically and would again cause the filter to be held open.
Another method of desensitizing the threshold control circuit employs a limiter circuit with a high ratio before the filter control block instead of a compressor with only a 2:1 ratio. This suffers not only the same undesirable side effect resulting with use of a compressor, but introduces a new problem in that, when the noise level of the audio source increases by about 10 dB, the filter will remain open and no longer operate.
It is therefore an object of the invention to provide a method of controlling the threshold of a single-ended noise reduction system with a DC control to have an intelligent threshold circuit that will determine the DC control signal fed to the dynamic filter and downward expander circuits based on determining the level of the background noise present in the audio signal. Another object of the present invention is to provide a circuit having large enough range of operation such that changing the audio source from an extremely noisy signal, such as a video cassette, to a far less noisy signal, such as a compact disc, will not inhibit proper tracking of the automatic threshold circuit or the performance of the noise reduction system. It is a further object cf this invention to provide a circuit which is capable of detecting the level of the noise floor even in the presence of an audio signal. Another object of this invention is to provide a method which may be implemented in Digital Signal Processing and modeled with a software algorithm.